This invention relates to well logging and more particularly to well logging methods and systems employing bender-type transducers in shear wave logging.
It is a well known practice to survey a well by acoustic logging techniques in which acoustic signals are generated and received by means of a logging tool run through the well. One acoustic logging technique involves the generation and reception of acoustic pulses and the determination of the travel time of the pulse signals between a transmitter and receiver or between spaced receivers. By this technique the velocity of sound through a subterranean formation may be determined in order to characterize the formation. Another acoustic logging technique involves amplitude logging in which the loss of amplitude of an acoustic signal as it travels between a transmitter and receiver, or between spaced receivers, is measured. Velocity and amplitude logging may be carried out separately or in combination, that is the logging tool may be equipped with appropriate circuitry to detect both the travel time of the acoustic signal and the loss in amplitude.
An acoustic signal may be transmitted through a subterranean formation in both compressional and shear (transverse) modes. Since a shear wave cannot be transmitted along the borehole through liquid therein, it has been proposed to transmit and receive shear wave by transducers positioned in contact with the borehole wall. For example, U.S. Pat. No. 3,949,352 to Vogel discloses a shear wave acoustic logging system employing transmitting and receiving transducers spaced in close proximity with one another and located within a transducer mounting pad which is pressed against the wall of the borehole.
While the Vogel procedure requires a direct coupling of the transmitting and receiving transducers to the borehole wall, shear wave logging employing so-called "indirect" excitation of the borehole wall is disclosed in Kitsunezaki, "A New Method for Shear Wave Logging", OYO Tecnical Note, Urawa Research Institute, October, 1978. In this procedure an electromagnetic transducer is "suspended" in water (the borehole fluid) and employed to generate an asymmetric wave pulse (asymmetric acoustic radiation pattern) through the borehole fluid into the formation. The transducer is oriented such that an excitation bobbin vibrates along an axis normal to the axis of the well. The asymmetric radiation pattern in the fluid becomes a compressional wave and a shear wave at the formation fluid interface. The resulting shear wave travels through the formation and crosses the formation fluid interface at a location near receivers. When in the fluid, the shear wave becomes an asymmetric radiation pattern which is detected at a plurality of receivers spaced longitudinally from the transmitter. The receivers take the form of geophone type detectors in a detector body suspended in the borehole fluid and having an apparent density adjusted to be the same as the density of the borehole fluid.